Kinetics of Hydrolysis of BCNU was studied at 37 degrees in presence of various nucleophiles at or near physiological pH's, to determine if nucleophilic catalysis occurs during the degradation of BCNU in the body. At pH 7.4, imidazole, serine, poly-L-lysine, methionine and TRIS had no catalytic effect on BCNU hydrolysis at 0.2 M concentrations, whereas stronger nucleophiles like iodide and bisulfite catalyzed the rate by about 20%. Sulfydryl nucleophiles with pKa at or below 8.5 (cysteamine, cysteamine ethyl ester) catalyzed the reaction by about 50% at 0.2 M concentration. When a disubstituted nitrosourea, MPCNU was used, its hydrolysis rate at pH 8.3 was increased 4-fold in the presence of 0.1 M cysteamine. It was concluded that strong catalytic activity of sulfydryl nucleophiles is partially masked by rather fast spontaneous hydrolysis of N3-protonated nitrosoureas and becomes more prominent with N3-dialkyl substituted nitrosoureas.